Cellulosic biomass is an abundant and sustainable natural resource. Its efficient utilization has long been the focus of research and development efforts with the aim to compete with and replace petroleum-based products. In addition to the production of bio-fuels (e.g., ethanol) that has proven to be successful,[1-4] the conversion of cellulosic biomass to high value-added chemicals is equally important.[5-9] In comparison with the conventional fermentation processes, catalysis would provide a greener and more cost-efficient route for producing chemicals of commercial interest from biomass.[4] There have been numerous studies on catalytic conversions of various carbohydrates to useful compounds or important chemical intermediates. For example, sorbitol and gluconic acid were produced from glucose or cellobiose by catalytic hydrogenation and oxidation respectively;[10-13] 5-hydroxymethylfurfural (5-HMF) and levulinic acid were prepared by catalytic dehydration of different carbohydrates;[14-17] methyl lactate was derived with high yield from sucrose, glucose and fructose using Sn-doped Lewis acidic zeolite catalyst;[18] catalytic isomerization between fructose and glucose has also been studied.[19,20] Despite the great success achieved with small-molecule biomass derivatives (e.g. mono- and disaccharides), the direct catalytic conversion of cellulose is still a challenge, mainly because cellulose is highly stable and insoluble in most solvents. To overcome this problem, ionic liquids are usually used as solvent for their special ability to dissolve cellulose,[21-23] or alternatively extreme conditions are applied to facilitate the reactions.[5,24-26]
Heteropoly acids (HPAs) have been used for cellulosic biomass conversion, where the function of HPA is to catalyze the hydrolysis of cellulose and the products are limited to glucose and fructose.[27-29] US Patent Application Publication No. 20110257448 and US Patent Application Publication No. 20110009614 disclose that HPA-supported noble metal catalysts can be used to convert biomass-derived oxygenated hydrocarbons to sugar alcohols and hydrocarbons through hydrogenation reactions. The inventors of the present invention recently reported a novel composite catalyst, cesium hydrogen phosphotungstate-supported Au (Au/Cs2HPW12O40), for highly selective and complete conversion of a disaccharide (cellobiose) to gluconic acid, where the phosphotungstate and Au nanoparticles provide solid acid sites for hydrolysis and redox sites for selective oxidation respectively[11]. However, using HPAs (without noble metal) as catalysts to produce chemicals of commercial interest from cellulosic biomass materials via successive hydrolysis and selective oxidation has not been reported in literatures. In particular, the direct production of glycolic acid, an important compound widely used in organic synthesis, biodegradable polymer synthesis, skin-care products, industrial rust removal and food processing, from cellulosic biomass materials has not been achieved.